1. Field of the Invention
The present invention relates to a thin film magnetic head, and more particularly to a composition of the thin film magnetic head used for magnetic disk apparatus in a computer system. The magnetic head of the invention can reduce spurious and transient noise pulses comprised in a reproduced signal during a read operation, and therefore is useful in field which requires a high reliability in the output signal from the magnetic head.
2. Description of the Related Art
With a trend toward higher recording density and larger storage capacity of a disk apparatus, a thin film magnetic head used therein is required to have high reliability and high performance in operation.
FIG. 1(a) is a perspective view of a thin film magnetic head, a portion thereof being removed for easy understanding, and FIG. 1(b) is a cross sectional view of this magnetic head. A substrate 1 is of a ceramic material such as Al.sub.2 O.sub.3 TiC. On substrate 1, a magnetic circuit is formed, which is composed of two magnetic films 2, namely, upper magnetic film 2a and lower magnetic film 2b. Two magnetic films are shaped in a manner that width at one end both films is made narrow and a small gap 6 is formed between the two ends. At the other opposite end portion, two magnetic films are made to contact with each other. Each of the magnetic films 2a and 2b is a single layer of permalloy and formed by depositing or plating the permalloy. A thin film coil 3 of copper is formed by plating and patterning before forming the upper magnetic film 2a, and the coil 3 is buried in an insulation layer 4 of thermosetting resin material, insulated from the magnetic films. The coils 3 surrounds the contacting portion of these two magnetic films 2. Two magnetic films 2 form a yoke of the magnetic circuit and the coil 3 interlinks the magnetic circuit. The coil 3 is provided with two leads wires 5 of thin film which are formed simultaneously with the coil 3 by plating copper. The gap 6 between the ends (poles) of magnetic films is buried with a sputtered aluminum oxide (Al.sub.2 O.sub.3) layer, and the entire surface is covered with a protective layer 7 (not shown in FIG. 1(a )). The magnetic head is arranged close to a recording medium 10 of disk apparatus during write and read operations.
In a write operation, a signal current responding to write information is input from lead wires 5 and flows through the coil 3, which generates a leakage flux at the gap 6, which magnetizes the rotating recording medium 10. In a read operation, recorded information on the magnetic recording medium 10 includes magnetic flux in the magnetic circuit of magnet films 2, and the flux further induces a signal current in the coil 3 interlinked with the magnetic flux.
The above structure is a general description of the thin film magnetic head structure. Further, assuming that an orthogonal coordinate system is given as illustrated in FIGS. 1(a) and 1(b), it is generally known that, during fabrication of the magnetic films, a magnetic anisotropy (easy axis of magnetization) is given in the Y-direction, because this enhances a permeability of the magnetic film in the X-direction (hard axis of magnetization) which is coincident with the flux direction induced during write and read operations. Thus the magnetic head of the prior art has improved an efficiency in the write and read operations.
However, in an actual fabrication, completed thin magnetic film does not show a simple pattern of magnetization. FIG. 2 illustrates an exemplary pattern of magnetization, in which only a magnetic film 2 is shown on the substrate 1 and other parts are removed for the purpose of simplicity. A plurality of magnetic domains are formed divided by domain walls, each domain having different direction of magnetization. The reasons therefor are that a length of the magnetic film is finite in the Y-direction, which causes demagnetization field in the magnetic film, and that, in order to reduce the magnetostatic energy caused by the demagnetization field, flux closure domains are generated in the magnetic film. As the result, flux closure domains 22 in FIG. 2 are formed in which direction of magnetization is perpendicular to those in main domains 21. In other words, when external magnetic field is removed, magnetic domains are formed in a manner that directions of magnetization therein form a closed contour. A 180 degree domain wall 201 is formed between adjacent main domains 21, and a 90 degree domain wall is formed between flux closure domain 22 and main domain 21.
When the thin film magnetic head of the prior art as described above is subjected to an application of signal magnetic flux, rotation of magnetization occurs in the main domain 21 further the domain wall 202 defining closure domain 22 deforms or moves. There is not a definite relationship between the movement of domain wall and the applied magnetic field. In case that there is a phase difference between the signal flux and the domain wall movement, a reproduced signal comprises so-called "wiggle" noises, and in another case that, though domain wall movement is restricted by defect or impurity comprised in the magnetic substance, the domain wall suddenly moves, which generates so-called "popcorn" noises in a reproduced signal.
FIGS. 3(a) and 3(b) illustrate an exemplary reproduced signal waveform the magnetic head of the prior art . "Wiggles" are observed on a trailing portion of the waveform in FIG. 3(a), and "popcorn" is observed on indefinite portion of the waveform in FIG. 3(b).
In order to reduce "popcorn" noise, Japanese Unexamined Patent Publication Tokukai-Hei 2-312005 discloses a heat treatment method, in which a magnetic head subassembly composed of magnetic films, a coil and etc. is subjected to the heat treatment of 270.degree. to 400.degree. C. before forming a protective layer 7 (shown in FIG. 1(b)). However, it has been found that this method can not always eliminate such noises. For example, when a permalloy (NiFe) layer is used as the magnetic film and its magnetostriction coefficient .lambda. is positive, the noise reduction can not be expected. In case when amorphous cobalt alloy layer is used as the magnetic film, either a heat treatment or a combination of the heat treatment and in-plane (hereinafter used to denote the parallel direction to the XY plane) magnetic anisotropy can not remove "popcorn" noises.